VISUAL SYMPTOM IDENTIFICATION OF GRAPEVINE LEAFROLLASSOCIATED VIRUS 3 IN RED BERRY CULTIVARS SUPPORTS VIRUS MANAGEMENT BY ROGUING
V.A. Bell, A.G. Blouin, D. Cohen, D.I. Hedderley, T. Oosthuizen, N. Spreeth, P.J. Lester, G. Pietersen
Grapevine leafroll-associated virus 3 (GLRaV-3) is the most serious virus in New Zealand and South African vineyards. Its negative influence on berry development is reflected on wine quality, thus making GLRaV-3 control a priority. In red berry cultivars, changes in leaf colour could be useful for the visual identification of GLRaV-3-infected vines with a view to roguing (removing) such vines. We tested the efficacy of visual diagnosis as a potentially cost-effective alternative option to the enzyme-linked immunosorbent assay (ELISA) that is usually used for this purpose. All the vines, or a subsection of vines, in multiple vineyards in New Zealand or South Africa where annual roguing was being performed, were evaluated with the two methods. Of the 114,782 vines assessed visually for symptoms and tested by ELISA, the two methods were in agreement for 114,701 (99.9%) vines, with only 81 vines showing differing results. In 11 of the 44 annual vineyard analyses, visual detection of symptoms was perfectly correlated with ELISA results (sensitivity 100%). The specificity of visual symptom identification compared with ELISA was higher than 99.7% in 43 of the 44 annual vineyard analyses. Symptoms as a predictor of negative ELISA proved to be above 97.5% in all 44 annual vineyard analyses but as a positive predictor, was 100% in 10 of 19 annual vineyard analyses where this could be determined. We conclude that for the red-berried cultivars in this study, visual assessment of foliar symptoms should be adopted as a cost-effective alternative to ELISA during implementation of roguing for GLRaV-3 control.